1. Field of the Invention
This invention pertains generally to the field of treating wafers with particle beams, and particularly to controlling the dosage of and ensuring the uniform distribution of the beam particles on to the wafers.
2. Description of the Prior Art
Various types of apparatus and methods have been developed for achieving a uniform ion dose on semiconductor wafers which are having ions implanted thereon by an ion particle beam. Several of such devices are shown in U.S. Pat. Nos. 3,778,626 to Robertson, No. 4,234,697 to Ryding and No. 4,517,465 to Gault, et al., the disclosures of which are herein incorporated by reference. As disclosed in the patent to Robertson, a rotating disk can be used to support wafers for batch processed ion implantation. To achieve a uniform dosage, the rotating disk must be scanned laterally along a radial pathway perpendicular to the beam by the ion beam at a velocity V=NI/2#rqD, where N is the number of single direction scans over the disk surface, I is the beam current where I/q is the beam particle current, r is the distance between the beam and the axis of rotation of the rotating disk, q is the expected charge per ion, and D is the final dose in ion particles per square area desired. The above-stated formula demonstrates that the linear translational velocity of the spinning disk must vary in proportion to the beam current I, and inversely with the distance between the ion beam and the axis of rotation of the disk.
This relationship has been put into practice in the past by various different methods and apparatuses. A variable pitch screw has been used to provide the l/r dependence and in addition, current on the disk itself has been measured and the velocity of the motor causing linear translational movement of the spinning disk changed to compensate for variations in that measured current. The patent to Robertson discloses measuring the current on the rotating disk, and measuring the distance between the rotational axis of the disk and the beam, and then driving the linear drive motor in proportion to I/r with a constant pitch linear drive mechanism. Both of these methods require that the current I be measured on the disk. Several problems occur in measuring the current I on the disk, though. A large disk is susceptible to the pickup of electromagnetic interference, and can have noise generated in the rotating current contacts. In addition, since the disk surface is not a flat smooth surface, it is difficult to prevent the escape from the rotating disk of secondary ions which are generated as the ion beam strikes the surface of the disk. As a result of these problems, the beam current I which is measured by these techniques might not be accurately indicative of the beam particle current. It is the beam particle current I.sub.p =I/q which is critical to determining ion particle dosage on the wafers.
These problems have been overcome in the past by using a disk having a rectangular slot cut therethrough along the radius of the disk, and using a Faraday cup behind the spinning disk to monitor the ion beam. Placing the Faraday cup behind the slotted spinning disk is a more accurate method of beam current measurement than is measuring the current on the disk. In addition, since the rotational velocity of the spinning disk varies in proportion to l/r for a disk spinning with a constant angular velocity, the charge collected in each current pulse in the Faraday cup varies according to I/r. This technique of gathering the current pulses in back of a slotted spinning disk with a Faraday cup was utilized in the patent to Ryding to control the linear translational velocity of the spinning disk. In the patent to Gault, et al., this technique was used to control a distance r between the ion beam and the axis of rotation of the disk, without the need for separately measuring the distance between the beam and the axis of rotation of the disk. With this method, the Faraday cup reading alone can be used to compensate for the variation in beam current I and distance r between the beam and the disk axis. Other techniques also were developed using a Faraday cup which would monitor the beam current I periodically by moving the rotating disk out of the way.
It has been found that another problem occurs with both the Faraday cup measurement techniques and with the disk current measuring techniques. Both methods may be susceptible to error due to charge exchange reactions between the ion beam and the molecules of the residual gas within the vacuum. In both methods, the implanted particle dose is calculated based on the beam electrical current and the expected charge state q of the beam. At low energies, the positively charged ion particles tend to pick up electrons to thereby become less positive or even neutral. At high energies the beam ion particles tend to strip to higher positive charge states. Under those conditions in which the residual gas pressure is relatively high, for example, when wafers coated with photoresist are implanted with ion beam particles, these charge exchange reactions often can lead to inaccurate doses. In such situations, the measured beam electrical current might not be a function of, or indicative of the beam particle current I.sub.p =I/q. Although the vacuum within the beam line might be in a good condition when the beam begins scanning the wafers, the wafer scanning by the particle beam causes the wafers to give off gaseous molecules which eventually permeate the beam line, thereby increasing the residual gas throughout the beam line.
A technique of passing a grounded wire through an ion beam to cause secondary electrons to be ejected from the wire, and collecting those emitted secondary electrons to display the shape of the beam is disclosed in U.S. Pat. No. 3,789,298 to Herb, the disclosure of which is herein incorporated by reference. In Herb a collector electrode is spaced outwardly from the scanning wire to collect the secondary electrons emitted by the scanning wire into a current. This secondary electron current then is directed to an oscilloscope and used to display the shape of the ion particle beam. The secondary electron current on the collector electrode has also been monitored by a metering circuit to determine relative beam current. For example, the secondary electron current has been monitored to determine whether or not the ion beam was still there, or whether its intensity had increased or decreased in relative value.
A need exists for an apparatus and method for uniform ion implantation dose control in which the beam current I measured is the function of the beam particle current I.sub.p =I/q, even when the residual gas pressure with the vacuum is relatively high.